Match local files to e621: MD5 lookups, manual links, batch scans

- MediaItem gains e621_match_status (unknown/matched/not_found/deleted)
  and e621_checked_at, backfilled for existing matched items.
- Server-side e621 client (apps/library/e621.py) using the user's stored
  credentials, throttled to 2 req/s, with typed errors.
- Matching service: MD5 lookup, manual post linking (flags MD5
  mismatches), unlink, metadata refresh, deleted-post detection.
- Detail actions POST /api/files/J-x/match/ and /unlink/ (uploader or
  staff only).
- Background library scans: MatchTask + /api/matches/ with missing/all
  scopes, progress polling, cancel and stale-task reaping; the scan
  counts toward the footer's Active Workers. Same pass available as
  manage.py match_e621 for cron.
- Library gains not_found/deleted status filters; the detail page adds
  an e621 match card (check / link by post ID / unlink) and the metadata
  card warns when a post was deleted on e621.

frontend/src/lib/types.ts

@@ -81,6 +81,8 @@ export interface MediaLocation {
 
 export type Rating = "" | "s" | "q" | "e";
 
+export type E621MatchStatus = "unknown" | "matched" | "not_found" | "deleted";
+
 export interface E621StoredPost {
   id: number | null;
   created_at?: string | null;
@@ -126,6 +128,8 @@ export interface MediaItem {
   uploaded_by_id: number | null;
   e621_post_id: number | null;
   e621_data: E621StoredPost | null;
+  e621_match_status: E621MatchStatus;
+  e621_checked_at: string | null;
   locations: MediaLocation[];
   raw_url: string;
   thumbnail_url: string;
@@ -185,6 +189,21 @@ export interface DownloadTask {
   updated_at: string;
 }
 
+export interface MatchTask {
+  task_id: string;
+  scope: "missing" | "all";
+  status: "pending" | "running" | "complete" | "error" | "cancelled";
+  total: number;
+  processed: number;
+  matched: number;
+  not_found: number;
+  deleted: number;
+  failed: number;
+  error: string;
+  created_at: string;
+  updated_at: string;
+}
+
 export interface E621IqdbCandidate {
   post_id: number | null;
   score: number | null;